1. A comparison of optical trains based on a GIMM
or a Dielectric Mirror final optic
HAPL San Diego, 9th Aug. 2006
Malcolm W. McGeoch
PLEX LLC
280 Albany St.
Cambridge MA 02139

2. Baseline HAPL final optical parameters:
2.5MJ at 5Hz, 40 illumination beams each 62.5kJ
2 Jcm-2 in optical distribution ducts.
Duct aspect ratio 6:1, each beam 3x18 beamlets
(area of one beam = 3x18x(0.24)2 = 3.1m2)
4. Focal length 39m (GIMM) or 42m (all-Dielectric case)
Vertical “slits” in blanket, total 0.6% of 4p
(slit size 1.35m high x 0.22m wide for GIMM case)
24cm x 24cm beamlet
from de-multiplex array

3. The final optics must function to attenuate the neutron flux:
target
3x1024 /beamline /FPY n
laser building
< 1x1019 /beamline /FPY
= < 1mrem/hr/day
at one day after shutdown
(M. Sawan, this meeting)
Attenuation factor of 106 !

4. We can encase the optical path with 1.5m - 3m of concrete
and attenuate using the angles that are needed in any case
for distribution around the sphere
target n
3x1024 neutrons/beamline /FPY
< 1x1019 /beamline /FPY
Concrete + ferritic steel + H2O coolant

5. 40 port arrangement: 3 tiers, 8 longitudes each hemisphere
View from above “North Pole”
Elevation

6. Uniformity for 9-element “basket”: N = 3,4,5 with g = 0.8, 0.9, 1.0

7. Elevation sketch of beams for all-dielectric final optics
(3J cm-2)

8. Plan sketch of beams in one quadrant for all-dielectric final optics

9. Radiation loads in all-dielectric design
Mirror location total n/FPY/cm2 n >0.1MeV/cm2 n>1MeV/cm2 gamma/cm2
M1 32.5m from target e e e e19
M2 9.5m from M1 6e e e e17
M3 15.5m from M2 1e16 5e15 3e e16
M4 1.6m from M3 2e14 3e e e14
M4 6m from M3 2e13 2e12 1e e13
Estimates courtesy of M. Sawan.

10. Radiation resistance of multilayers compared to requirements
Category: layer mixing reflectivity damage resistance
Dose:
1e19cm OK OK (vis*) ?
(4% of FPY)
1e20cm OK anneal? ?
(44% of FPY)
1e21cm nm? anneal? ?
(4 FPY)
*I. I Orlovskiy and K. Yu. Vukolov, “Thermal and neutron tests
of multilayered dielectric mirrors” Fus. Eng. Des. 74, (2005)

11. Mixing of layers was thought (*) to be a showstopper for
Layer mixing? (248nm mirror layers are about 35nm thick)
Mixing of layers was thought (*) to be a showstopper for
dielectric mirrors, but data on irradiated multilayers for
X-ray optics and superconductors eases this concern
(*) R. L. Bieri and M. W. Guinan, “Grazing incidence metal mirrors as the final
elements in a laser driver for inertial confinement fusion”, Fusion Technology
(1991).

12. Exposure of XUV mirrors to 1.1e19cm-2 (1-2MeV neutrons)
low index: Si or B4C, or C
high index: Mo or W
N bilayers
“d” spacing
S. P. Regan et al. “An evaluation of multilayer mirrors for the soft X ray
and extreme ultraviolet wavelength range that were irradiated with neutrons”
Rev. Sci. Instrum. 68(1), (1997)

13. XUV mirrors tested by Regan et al. (exposure temp 270-300C)
Composition “d” (nm) substrate N
Mo/Si 8.78nm Zerodur 50
W/B4C nm Si wafer 100
W/C 2.53nm Si wafer 100
Mo/Si 18.7nm Si wafer 25
Results:
Mo/Si mirrors had slight shift in peak L and slight reflectivity
decrease, exactly consistent with C known thermal effects.
W mirrors had opposite shift in peak L and slight increase in reflectivity,
again consistent with known thermal effects.
No effects attributable to fast neutron exposure of 1.1e19cm-2 (10-2 dpa/atom)

14. Layered superconductors have been tested for fusion reactor use
2nm AlN layers
9 - 26nm NbN layers (not to scale)
20 bilayers
R. Herzog et al. “Radiation effects in superconducting NbN / AlN multilayer films”
J. Appl. Phys. 68, (1990).
Results: Jc>108Am-2 at 4.2K and 20T, before and after irradiation
No degradation of 2nm AlN layers at 1x1019 neutrons/cm2 (>0.1MeV)

15. Lack of layer mixing is consistent with expectations
XUV reflectivity and enhanced Jc are both sensitive to the roughness
of the surfaces of layers, and the above results both indicate less than
about 1nm of induced roughness after irradiation to 10-2 dpa.
Bieri and Guinan (*) estimated mixing of roughly 3nm/(dpa)1/2, where
1dpa = 5x1020 (14MeV) n/cm2 for most dielectrics
For “low” doses (much less than1 dpa), this diffusion approach does not apply (for example, at 10-2 dpa, 99% of layer remains undisturbed).
A more critical test will occur at > 0.1dpa (5x1019 neutrons cm-2)
(20% of FPY)
(*) R. L. Bieri and M. W. Guinan, “Grazing incidence metal mirrors as the final
elements in a laser driver for inertial confinement fusion”, Fusion Technology
(1991).

16. Dielectric mirrors are being evaluated for the final optic
30 layers = 1.9µm …short absorption path
Neutron-stable sapphire substrate
500C anneal cycle removes
neutron-induced absorption

17. Neutron irradiation produces optical absorption in dielectrics

18. 248nm neutron-induced absorption is annealed out at 500C
Sample 0.6cm SiO2, 6.6e20 neutrons cm-2.
--> post anneal loss in 2µm dielectric path is negligible
Data from J. Latkowski, HAPL meeting, Rochester, Nov. 2005

19. Planned neutron irradiation of mirrors
All the reported multilayer exposures have been to about 1019/cm2
The key near term need is for exposure to 1020/cm2 (44% FPY)
A variety of KrF mirrors and GIMM quality aluminum mirrors will be
exposed to up to 1021 neutrons cm-2 at ORNL (>1FPY).
Some mirrors will be irradiated at high temperature (250C and 500C)
(K. J. Leonard et al. 14th laser IFE progm. wkshp, ORNL, March 2006)

20. GIMM design options (slide courtesy M. Sawan)
Two options considered for GIMM materials and thicknesses
Both options have 50 microns thick Al coating
Option 1: Lightweight SiC substrate
The substrate consists of two SiC face plates surrounding a SiC foam with 12.5% density factor
The foam is actively cooled with slow-flowing He gas
Total thickness is 1/2"
Total areal density is 12 kg/m2
Option 2: Lightweight AlBeMet substrate
The substrate consists of two AlBeMet162 (62 wt.%Be) face plates surrounding a AlBeMet foam(or honeycomb) with 12.5% density factor
Total thickness is 1"
Total areal density is 16 kg/m2

21. Radiation loads in SiC GIMM design
5Jcm-2
Mirror location total n/FPY/cm2 n >0.1MeV/cm2 n>1MeV/cm2 gamma/cm2
GIMM(M1)24m from target e e e e20
M2 14.9m from M e18 9e17 8e e17
M3 1.6m from M2 2e16 6e e e16
M3 6m from M2 2e15 4e14 2e e15
Estimates courtesy of M. Sawan.

22. Unknowns remain for both the GIMM and dielectric approaches:
After neutron exposure:
Laser-induced damage has not been measured in either case
Reflectivity at 248nm (KrF) has not been measured in either case
Substrate optical quality has not been measured (except to 1019 cm-2)
Other considerations:
The GIMM requires polarized light => less random illumination
The GIMM area is 14m2 vs dielectric area 1.9m2 => cost is high
The neutron maze is much more effective in the dielectric case